1. Field of the Invention
The present invention relates to a semiconductor laser diode and a method for fabricating the same, and more particularly, to a nitride semiconductor laser diode having a buried heterostructure (BH) for maximizing an electric optical confinement effect in a lateral direction using a selective growth technique, and a method for fabricating the same.
2. Description of the Related Art
In general, since semiconductor laser diodes are comparatively small and threshold current for laser oscillation of the semiconductor laser diode is smaller than that of a conventional laser device, semiconductor laser diodes have been widely using as devices for high speed data transmission or high speed data recording and reading in communications or players in which optical discs are used.
In particular, nitride semiconductor laser diodes generate laser with wavelength from green to an ultraviolet region, being widely applied in high-density optical information storing and reproducing, high-resolution laser printers, and projection TVs.
Likewise, as the semiconductor laser diode is widely used in various fields, a semiconductor laser diode having low threshold current and high efficiency is emerging, and a ridge-type semiconductor laser diode and a semiconductor laser diode having a buried heterostructure (BH) are representative of semiconductor laser diodes having low threshold current and high efficiency.
FIG. 1 is a cross-sectional view of a conventional ridge-type semiconductor laser diode and shows a semiconductor laser diode having a ridge so as to reduce threshold current for laser oscillation and realize the stability of a mode.
Referring to FIG. 1, the ridge-type semiconductor laser diode has a structure in which an n-type cladding layer 13, an n-type waveguide layer 15, an active layer 17, a p-type waveguide layer 19, a p-type cladding layer 21 are sequentially stacked on a substrate 11. The reflective indices of the n-type and p-type cladding layers 13 and 21 are lower than the reflective indices of the n-type and p-type waveguide layers 15 and 19, and the reflective indices of the n-type and p-type waveguide layers 15 and 19 are lower than the reflective index of the active layer 17. The p-type cladding layer 21 has a ridge 21a projected in an upper middle portion of the p-type cladding layer 21. The ridge 21a of the p-type cladding layer 21 confines current injection, thereby limiting a resonance region for laser oscillation in the active layer 17. A capping layer 25 is stacked on the top surface of the ridge 21a of the p-type cladding layer 21. The top surface of the p-type cladding layer 21 excluding the ridge 21a is covered with a current confinement layer 23, and the top surface of the capping layer 25 excluding a middle portion, which becomes a current path, is covered with the current confinement layer 23. A p-type electrode 27 is formed in the middle portion of the top surface of the capping layer 25 and on the top surface of the p-type cladding layer 21, and an n-type electrode 29 is formed on the bottom surface of the substrate 11.
The ridge-type semiconductor laser diode is fabricated by re-growing the current confinement layer 23 after stacking and growing the n-type cladding layer 13, the n-type waveguide layer 15, the active layer 17, the p-type waveguide layer 19, the p-type cladding layer 21, and the capping layer 25 on the substrate 11 and forming a ridge structure through a predetermined etching process.
In the ridge-type semiconductor laser diode, due to the ridge structure, current injection is confined causing the width of resonance to be limited, and thus an optical mode is improved a little compared with a conventional non-ridge structure, and threshold current for laser oscillation is lowered.
FIG. 2 is a schematic cross-sectional view of a semiconductor laser diode having a buried heterostructure (BH). Referring to FIG. 2, an active layer 33 is formed on the top surface of an n-type compound semiconductor layer 31 having a mesa structure, and p-type and n-type current blocking layers 35 and 37 for confining current and light are formed at both sides of the mesa structure including the active layer 33. A p-type compound semiconductor layer 39 is formed on the active layer 33 and the current blocking layers 35 and 37. A p-type electrode 41 is formed on the top surface of the p-type compound semiconductor layer 39, and an n-type electrode 43 is formed on the bottom surface of the n-type compound semiconductor layer 31.
The semiconductor laser diode having a BH as above is fabricated by growing the active layer 33 on the n-type compound semiconductor layer 31 through liquid phase epitaxy (LPE) or metal organic chemical vapor deposition (MOCVD), forming the mesa structure through a predetermined etching process and re-growing the current blocking layers 35 and 37 and the p-type compound semiconductor layer 39.
The semiconductor laser diode having a BH has advantages such as small threshold current and stable oscillation horizontal mode characteristics, because the up and down and right and left sides of the active layer 33 are surrounded by the n-type and p-type compound semiconductor layers 31 and 39 and the current blocking layers 35 and 37, respectively, and thus it is known that the semiconductor laser diode having a BH has performance higher than the ridge-type semiconductor laser diode.
However, it is not easy in the nitride semiconductor laser diode to perform etching and re-growth processes for growing the BH, unlike other III-V group semiconductor laser diodes, and thus the nitride semiconductor laser diode is still dependent on a basic ridge structure.
This is the reason the nitride semiconductor layer generates laser with wavelength from green to ultraviolet region, whereas there are problems on lattice constant inconsistency, high melting point, and hardness of a material when constituting a composite material, and thus there are difficulties in making variation such as re-growth when growing the structure of a laser diode, and in a process such as a wet etching process.
However, the conventional ridge-type nitride semiconductor laser diode has problems such as instability of optical mode characteristics due to the shape and depth of etching, increase in threshold current due to weak index-guide, and deterioration of long-term reliability due to the exposure of an etching surface. Thus, in order to fabricate a laser diode having low threshold current and high output required in high-density optical recording and reproducing, the development of a nitride semiconductor laser diode having an improved structure such as the BH is required.